The increasing availability and use of semiconductor light sources and particularly laser diodes which emit in the near infrared region of the electromagnetic spectrum has led to a need for high quality photographic materials which are sensitive in this region, especially from 650 nm to 850 nm.
In order to improve the image sharpness of photographic materials it is customary to incorporate a dye in one or more layers of the material, the purpose of which is to absorb light that has been scattered within the coating and would otherwise lead to reduced image sharpness. Dyes used for this purpose are known as antihalation dyes if incorporated in a separate backing layer or underlayer and as acutance dyes if incorporated into the light sensitive layer itself.
It is usually essential that antihalation or acutance dyes should completely decolourise under the processing conditions of the photographic material concerned. In the case of photothermographic materials which are processed by simply heating for a short period usually between 100.degree. C. and 200.degree. C. any antihalation or acutance dyes used must decolourise thermally.
Various thermal-dye-bleach systems are known in the prior art including single compounds which spontaneously decompose and decolourise at elevated temperature and combinations of dye and thermal dye bleaching agent which together form a thermal-dye-bleach system.
U.S. Pat. Nos. 3,609,360, 3,619,194, 3,627,527, 3,684,552, 3,852,093, 4,033,948, 4,088,497, 4,196,002, 4,197,131, 4,201,590 and 4,283,487 disclose various thermal-dye-bleach systems which absorb principally in the visible region of the electromagnetic spectrum and as such they are not readily adaptable for use as near infrared absorbing constructions. No indication or examples are given of near infrared absorbing thermal-dye-bleach systems.
A variety of thermal base-generating agents are known and have been used in photothermographic materials. However, in all cases in which thermal base-releasing agents have been incorporated into photothermographic constructions in the prior art the purpose has been to increase the alkalinity of the medium during thermal processing and to promote the development reaction. Thermal base-releasing agents have been used thus in photothermographic materials of both the diazo type and silver based materials.
Our co-pending European Patent Application No. 89312472.7, filed Nov. 30, 1989, discloses the use of certain polymethine dyes for infra-red antihalation in both wet-processed and dry-processed photographic materials. The dyes bleach completely during wet-processing, but remain unbleached after dry-processing. This is acceptable for some purposes because the dyes have a relatively small component of their absorption in the visible region, which can be masked for example, by using a blue-tinted polyester base. For most applications, however, it is preferable that the dyes bleach completely during dry-processing, leaving no residual stain.